With increasing development of science and technology, a variety of electrical appliances are frequently used. As known, power supply is necessary for normal operation of the electrical appliance. However, when AC power is supplied to the electrical appliance, the current provided by the AC power usually contains noise because of the power supply or the operations of some components (e.g. high-frequency transformers, stray capacitors or parasitic capacitors). The noise is also referred as electromagnetic Interference (EMI).
When an AC power is used, the generated noise includes differential mode noise or common mode noise may occur. An electromagnetic interference filter (also referred as an EMI filter) is used as a first defense line for protecting the power source from electromagnetic radiation. Generally, the EMI includes a choke and a capacitor. The choke is an inducting element for inhibiting noise generation. Moreover, the choke includes at least one winding coil, a magnetic core assembly, and a bobbin for winding the winding coil.
Generally, the EMI filter is equipped with a choke for suppressing the noise. The conventional EMI filter with a one-stage filtering function includes two filtering circuits for receiving a dual-input power (e.g. a first input power and a second input power). For suppressing the noise, the conventional EMI filter includes two independent chokes for receiving the dual-input power. The two chokes are connected with the two filtering circuits, respectively. Consequently, the corresponding choke can suppress the noise contained in the input power, which is received by each filtering circuit. However the use of the two chokes increases the fabricating cost of the EMI filter. Moreover, if the voltage values received by the two input terminals of the EMI filter are different, the current inputted into the positive terminal of each filtering circuit and the current outputted from the negative terminal of each filtering circuit are not equal. Meanwhile, the choke is in an unbalanced status and subject to saturation, and thus the impedance of the choke is very small. Consequently, the current flowing through the choke becomes very large. Under this circumstance, the possibility of burning out or damaging the components of the choke and EMI filter will be increased.